update load balancing output to use utils::logmesg() and {fmt}

src/imbalance_var.cpp

@@ -20,6 +20,7 @@
 #include "variable.h"
 #include "memory.h"
 #include "error.h"
+#include "fmt/format.h"
 
 using namespace LAMMPS_NS;
 
@@ -88,7 +89,7 @@ void ImbalanceVar::compute(double *weight)
 
 /* -------------------------------------------------------------------- */
 
-void ImbalanceVar::info(FILE *fp)
+std::string ImbalanceVar::info()
 {
-  fprintf(fp,"  weight variable: %s\n",name);
+  return fmt::format("  weight variable: {}\n",name);
 }